1. Field of the Invention
The present invention relates to a filter circuit and radio communication apparatus, and to a filter circuit for band limitation connected to a post-stage of a power amplifier for use in a transmission section of a communication apparatus using radio transmission, for example.
2. Related Art
As shown in FIG. 26, a conventional filter circuit is configured by cascade connection of resonators 1107(1) to 1107(n). An equivalent circuit of each of the resonators consists of an inductor and a capacitor, and a resistance is added when the effect of loss is considered. A resonance frequency of the resonator in the case of including no resistance is given by the following expression:F0=(L×C)−1/2 
where “L” and “C” are respectively an inductance and a capacitance of the resonator. In the filter circuit, the resonators are connected in cascade and inter-resonator coupling coefficients (m12, m23, . . . , Mn-1,n in FIG. 26) indicating coupling amounts of the resonators and a value of an external portion Q (Qe in FIG. 26) indicating an amount by which the resonators are excited in input/output portions are adequately determined, so that a pass frequency range and a stop band attenuation as a filter circuit can be determined. Reference numeral 1101 denotes an input terminal, and reference numeral 1106 denotes an output terminal. Since a current is propagated through each resonator in the filter circuit where the resonators are connected in cascade, currents of all frequency components pass through the resonators. Therefore, in a case where resonators are configured using a material having a limitation on a value of current per unit area which can pass in a superconducting state, such as a superconductor, power handling capability of each of the resonators is an important parameter for passing large power through the filter circuit, and a method is under study in which countermeasures are taken to prevent concentration of currents in the resonators by applying a disk shape or wide lines or some other means so as to improve power handling capability. However, there is a problem with the superconducting resonator in that current concentration occurs to a large degree since a value of the external portion Q is very high and it is thus not possible to obtain large power handling capability merely by devising the resonator shape.
Meanwhile, there is a method of configuring a filter circuit by parallel connection of resonators as a method of dispersing power into each resonator in the filter circuit to realize filter characteristics as shown in FIG. 27 (JP-A 2001-345601 (Kokai), JP-A 2004-96399 (Kokai)). With such parallel configuration of resonators, inputted power is distributed into resonators 1108(1) to 1108(n), to improve a power handling capability as a whole. For making parallel configuration of resonators, the resonators are configured so as to each have a different frequency (f1, f2, . . . , fn in FIG. 27) and synthesized such that adjacent resonators having resonance frequencies are in reverse phases to each other, thereby to realize the filter characteristics. In the figure, “-” in reference symbol “-m2” denotes reverse phase coupling. There is a method of combining a superconducting filter with a normal conducting filter in a filter configuration using the above-mentioned configuration (Japanese Patent No. 3380165, JP-A 11-186812 (Kokai)). In Japanese Patent No. 3380165, a superconducting filter and a normal conducting filter are arranged in parallel. However, there is a problem in that, when a large current is inputted, power is divided into each filter and then separated only into power to be reflected and power to pass in each filter, thereby requiring the superconducting filter to also have large power handling capability.